For the on-line uninterruptible power supply (UPS) systems, there are in general three working modes: the on-line mode, the battery mode, and the bypass mode. In the dual-mode, a power supply is offered to the load by a battery and through a DC—DC converter and an inverter. Usually, the DC voltage values offered by the battery are transformed and boosted to the DC voltage values required by the DC Bus through the DC—DC converter, and the DC voltage values of the DC Bus are transformed to the AC voltage outputs through the inverter.
Limited by the voltage values of the single battery, the battery set of the UPS system is usually composed by series connected batteries. For achieving a higher reliability of the system, the number of batteries coupled in series is relatively less so as to have the relatively lower output voltage values of the battery set. Compared with the converters having the half-bridge and the full-bridge configurations, the push-pull converters have the relatively lower conducting losses so as to be employed frequently in those UPS systems having the medium or low power values.
To increase the power densities of the UPS systems, having the relatively higher working frequencies of the power supply system is a necessary choice. To let the switches of the power system work in a relatively higher switching frequencies, the losses of the switches must be decreased. Since the capability of a battery to supply the electrical power is limited, thus the requirements regarding the efficiencies of the systems are relatively higher. Base on the above-mentioned reasons, researches regarding the soft-switching circuits are becoming more and more important.
In the prior arts, there are three methods for soft-switching the push-pull converters, and the circuit configurations of which are described as follows:    1. ZVS Push-Pull Converters            Please refer to FIG. 1, it shows the schematic circuit diagram of a ZVS push-pull converter having secondary side synchronous rectifier circuit. The exciting current of the transformer, the proper arrangement of the main switches on the primary side of the transformer, and the driving signals of the synchronous rectifier circuit on the secondary side are employed to accomplish the zero-voltage switching of all the switches in this circuit.        The advantages of this ZVS push-pull converter are that there is no special requirement regarding the duty ratios and this circuit can be employed in a pulse width modulation (PWM) mode operation. But, the number of switches is increased and the control of the circuit is relatively more complex. Thus, this circuit is not desirable for the cost-concerned applications.            2. ZVS LCL-Resonant Push-Pull Converters            Please refer to FIG. 2, it shows the schematic circuit diagram of a LCL-resonant push-pull converter. In the “LCL”, the first L (as L1 in FIG. 2) indicates the leakage inductance limited to the secondary side of the transformer. The unique features of this circuit include that the CL unit is located after the rectifier diodes, and the frequency of the resonant angle of the LCL resonant circuit is:        
  ω  =                                          L            1                    +          L                                      L            1                    ⁢          LC                      .                  Due to the applications of the exciting current of the transformer and the buffing of the drain-source capacitance of the switches, the switches at the primary side of the transformer could be operated under the zero-voltage switching status. And the resonant capacitor can be employed to snub the reverse recovery of the diode.        Although this kind of ZVS push-pull converters can be employed to accomplish the ZVS of the main switches, but the requirements regarding the duty ratios of these converters are relatively more rigid so as to desire the fix-frequency open-loop control having relatively larger duty ratios and a pre-determined resonant frequency twice the switching frequency. If these two conditions are not fulfilled, the effectiveness of the resonance will be influenced dramatically. Besides, there are tradeoffs regarding the choosing of the CL elements of these converters even though the switching frequency and the leakage inductance are given. If the capacitance values of the C element are relatively higher, the voltage ripples on the resonant capacitor and the voltage stresses on the output rectifier diodes are relatively lower, but the current ripples on the C and L elements are relatively higher. On the contrary, choosing relatively higher inductance values of the L element would generate the opposite results. Furthermore, the properly choosing of the depth of the resonance is relatively important. Otherwise, the conduction losses could be relatively higher even though the turn-off losses would be relatively lower. Since the resonant currents flow through the load, the ripples of the output voltages of this circuit are relatively hard to control.            3. ZVCS LC Resonant Push-Pull Converters            Please refer to FIG. 3, it shows the schematic circuit diagram of a ZVCS LC resonant push-pull converter. In its' resonant circuit, the L represents the leakage inductance counted on the secondary side of the transformer. The driving signals of the main switches are the fix-frequency pulses having the duty ratios slightly less than 0.5. The quality factors of the resonant circuit should be lower enough to let the resonant current iR operate under the discontinuous conduction mode.        The exciting current of the transformer and the drain-source capacitance of the switches are employed to accomplish the zero-voltage turn-on of the main switches of the primary side of the transformer, and the zero resonant current ir of the LC resonant circuit of the secondary side of the transformer is employed to accomplish the zero-current turn-off of the main switches.        These converters are different from the ZVS LCL resonant push-pull converters. Due to most of the resonant currents flow through the output capacitor, it is relatively easy to control the ripples of the output voltage of this circuit. But, the properly choosing of the depth of the resonance of these converters is relatively important. Otherwise, the conduction losses could be relatively higher.        
Keeping the drawbacks of the prior arts in mind, and employing experiments and research full-heartily and persistently, the primary side ZVS push-pull converter having relatively less losses is finally conceived by the applicants.